Why does a radio still work inside a metal box?

In summary, a commercial radio placed inside a metal tin with a tight lid may work when receiving an AM station (1 MHz) due to the frequency-dependent penetration depth of RF waves. However, it may only produce static when switched to FM (100 MHz) due to the smaller skin depth at higher frequencies. Grounding the tin may improve reception, but the use of a ferrite rod antenna may also play a role. The presence of a narrow slot around the lid's closure may allow the radio to work at MF, and the length of the slot is more important than its width. The hissy static heard may be caused by a variety of noise sources, including cosmic rays, solar wind, and RF radiation from nearby devices.
  • #36
Paul Uszak said:
Some contributors have suggested that the hiss is thermal noise in the early stages of the receiver. I take this to be stochastic Nyquist noise. If so, surely its amplification should also be random...
The amplitude of thermal noise (AM) is pink but the phase noise of the 1'st LO (FM) is not.
Also, any AC coupled noise will not be truly random as a big high or a string of highs will automatically adjust the bias to produce more lows to compensate. You will see the time constant of the AC coupling in your assumed-random data.
 
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  • #37
There's a difference between random noise and white noise and uncorrelated noise. If you are looking to do encryption, you "need" the third. (Technically white noise is uncorrelated, but that is by definition. Proving any supposed white noise is indeed uncorrelated would be a job.)

As I understand the encryption problem, even a tiny bit of correlation effectively cuts your key length in half. One solution would be to double the key length.

OTOH, if you are trying for a one time pad (IMO the way to go), you can't have any correlation.

I have serious doubts about all commercial encryption systems. Anyone who cracks such systems has every motivation to keep that fact a secret.
 
  • #38
Jeff Rosenbury said:
I have serious doubts about all commercial encryption systems. Anyone who cracks such systems has every motivation to keep that fact a secret.
Autocorrelation is not important when the PRBS covers all states and is longer than a human lifetime. The weakness of cryptosystems has been in the human component ever since the advent of multiple parallel PRBSs that are XORed together, all with long and different repeat times.
 
  • #39
Baluncore said:
Autocorrelation is not important when the PRBS covers all states and is longer than a human lifetime. The weakness of cryptosystems has been in the human component ever since the advent of multiple parallel PRBSs that are XORed together, all with long and different repeat times.
What you say is completely true -- unless someone smarter than you comes along.
 
  • #40
Jeff Rosenbury said:
What you say is completely true -- unless someone smarter than you comes along.
Some very smart people have been using LSFR keystream generator systems for a very long time.
https://www.nsa.gov/about/_files/cryptologic_heritage/publications/misc/tsec_kw26.pdf
 
  • #42
I once broke the RSA system. (I found a way to trade memory for execution speed in a brute force attack. It only worked for short key lengths.) I approached the company; they weren't interested. I approached the NSA; they weren't interested. I couldn't find any ethical way to use this hack. So I dropped it.

My point is that no one wants to fix a broken encryption system. The people using it don't want to spend the money and effort to find a better system. The company which sells the broken system prefers no one know it is broken. And anyone who breaks it wants to keep the break a secret (I'm looking at you NSA:oldshy:). So I expect most of the systems sold have been broken (more than once), not because they are bad systems, but because no one wants to deal with breaks.

I do agree that most breaks are human breaks or partial breaks (like mine), but so what? If no one is motivated to fix the systems, they are still broken.
 
  • #43
Paul Uszak said:
The magnet experiment, and some other actions suggested by contributors:-

The tin IS magnetic, attracting a magnet well
The walls are 0.25mm thick (including paint)
The inside of the tin is unpainted

This is the tin and radio (should have posted a photo earlier :frown: )

View attachment 84698
http://argos.scene7.com/is/image/Argos/5003820_R_Z002A_UC1051961?$TMB$&wid=312&hei=312

I grounded the tin to the Earth wire from a UK mains electrical socket, and repeated the reception test. I filed off the paint where the wire touched the tin. Results:-
FM: Total static. No discernible change with /without Earth wire. No Gaga.
AM: Tinny but recognisable radio station. No, repeat no discernible change in pitch or volume with /without Earth wire. Surprised.
To eliminate a possible entry around the lid you can try putting some copper tape around the lid seam.

Another possibility is the AM signal is not going through the tin, but is being re-radiated inside the box, with the tin box acting as a passive repeater. Nothing much you can do about that, I think.

Have you tried removing the AM antenna from the radio?
 
  • #44
Tom_K said:
To eliminate a possible entry around the lid you can try putting some copper tape around the lid seam.

Another possibility is the AM signal is not going through the tin, but is being re-radiated inside the box, with the tin box acting as a passive repeater. Nothing much you can do about that, I think.

Have you tried removing the AM antenna from the radio?

It's not so much it's being re-radiated. Even if the box had perfect E field shielding the H field would still not be totally excluded by absorption or reflection from skin-effect due to the low relative permeability and low cross-section of the metal at 1mhz. The actual loss might be much less than a typical skin-effect loss calculation. Usually the internal ferrite loop antenna (that's sensitive to the magnetic field component of the AM signal) is part of the frequency tuning circuit in a small AM radio so it can't be removed.
 
  • #45
Audacity (http://audacity.sourceforge.net/) is available for Linux & Windows. It's free. It can perform frequency analysis, autocorrelation and plot a spectrogram.
 
  • #46
I don't really have any copper tape to hand for this unfortunately. And, the AM aerial is built into the radio so can't be removed.

I think that we have to conclude that I'll find it difficult to effectively screen an AM radio. I'll have to restrict my entropy source experiments to FM frequencies only. That should be all right. I can procure 1mm sheet copper so I can solder up a better box.
 
  • #47
Paul Uszak said:
I think that we have to conclude that I'll find it difficult to effectively screen an AM radio. I'll have to restrict my entropy source experiments to FM frequencies only.
How are you going to get the power into the box and the audio electrical noise out of the box without letting RF in ?
 
  • #48
Baluncore said:
How are you going to get the power into the box and the audio electrical noise out of the box without letting RF in ?

To be honest, I don't know exactly but have some ideas and a blind faith that this is a common surmountable scenario in electronics :rolleyes:.

I also don't know whether this should be broken off into another PF question (I'm a Newbie).

In the case that moderators judge this thread continue... I believe that there is an opening to frequency ratio that governs the maximum allowable aperture through an EMI shield. I intend to exit the audio with a shielded cable and metal jack plug. 5VDC power will come from a PC but it's case will have been compromised with a large (300mm diameter) hole in one side; it's a modded PC case. I can shield that cable too. I'm assuming that it's only frequencies around 100MHz that the radio can decode in FM mode, thus other spectra are largely irrelevant.

My knowledge peters out at this point. I need advice as to bonding the copper box, PC case, 0VDC rail, audio cable and power cable shields. There's a lot of literature that I've not yet assimilated, or indeed understood. The fact that drives me is that the static audio need not be white (or any other shade) noise, and some interference is acceptable. The absolute and defining requirement is that the noise waveform in its entirety be unpredictable over a continuous sample period such as 1 second. It mustn't be all the same output. The waveform can be any shape. It would actually be fine if it was a majority of 50Hz mains hum, as long as there was an unpredictable component superimposed on top that can be identified with digitisation (16 bit resolution).

I'd like to explore my box's construction further, perhaps as another question..?
 
  • #49
I do not like your chances of success with a radio in a box.

meBigGuy gave a circuit in post #28 that would do the job. The great advantage is that it has no antenna.
It will need a comparator to generate a sequence of binary data bits.

In my opinion, with hardware you are better going for a long pseudo random binary sequence which does not need an AC coupled comparator.

A pseudo random binary sequence can also be generated by software. Most language packages have an excellent random number generator, usually far better than any electronic noise source. Those code generators are available free on the web, you only need to select an initial key to get a random sequence.
 
  • #51
I'll say it again, loudly --- A RADIO IS A TERRIBLE SOURCE FOR RANDOM NOISE (in general, and also for the reasons I stated). If you do not look at the noise source's spectrum and autocorrelation and do what ever is needed to make it clean, you are not doing your job. PERIOD! You are totally guessing, and are likely wrong in a big way.

The simple circuits I posted, the single chip digital pseudo random generator, and many more google-findable noise sources are all wiser choices, simpler, cheaper and probably outperform anything you can do with a radio.

A radio is a complex design with unintended feedback, power supply noise, many components with possibly long term correlated effects. The list goes on and on.

I really don't understand why you are so fixated on the radio and blind to the superior results of nearly any credible design.
 
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  • #52
meBigGuy said:
I'll say it again, loudly --- A RADIO IS A TERRIBLE SOURCE FOR RANDOM NOISE (in general, and also for the reasons I stated). If you do not look at the noise source's spectrum and autocorrelation and do what ever is needed to make it clean, you are not doing your job. PERIOD! You are totally guessing, and are likely wrong in a big way.

The simple circuits I posted, the single chip digital pseudo random generator, and many more google-findable noise sources are all wiser choices, simpler, cheaper and probably outperform anything you can do with a radio.

A radio is a complex design with unintended feedback, power supply noise, many components with possibly long term correlated effects. The list goes on and on.

I really don't understand why you are so fixated on the radio and blind to the superior results of nearly any credible design.
I agree. I lost track of what the experiment was all about. Initially, I thought it was all about shielding RF, and why an AM radio signal was harder to shield than FM.

If it is only about generating random noise in the audio range, he can probably do no better than what is available here.

And everything you want to know about noise and noise measurements is here.
 
  • #53
Paul Uszak said:
The magnet experiment, and some other actions suggested by contributors:-

The tin IS magnetic, attracting a magnet well
The walls are 0.25mm thick (including paint)
The inside of the tin is unpainted

This is the tin and radio (should have posted a photo earlier :frown: )

View attachment 84698
http://argos.scene7.com/is/image/Argos/5003820_R_Z002A_UC1051961?$TMB$&wid=312&hei=312

I grounded the tin to the Earth wire from a UK mains electrical socket, and repeated the reception test. I filed off the paint where the wire touched the tin. Results:-
FM: Total static. No discernible change with /without Earth wire. No Gaga.
AM: Tinny but recognisable radio station. No, repeat no discernible change in pitch or volume with /without Earth wire. Surprised.
Tell us more about the wire that goes into the box.
 
  • #54
arydberg said:
Tell us more about the wire that goes into the box.

The wire is shown in the picture you posted it looks like flat flex cable of 2 or 3 or 4 wires. Is it used for a speaker or power? it may be acting as an antenna and bringing signals into the box.
 
  • #55
Hiss

Tom_K said:
I lost track of what the experiment was all about.

Sorry about that. Two of my questions were merged together for me at the start and thus unfortunately two threads run throughout this post (Hiss & shielding)

With respect, I'm not blind to alternative sources of entropy, and I'm quite familiar with their performance characteristics. I am committed to hardware generation, not pseudo random. I like the idea of using a radio. After all fish tanks, lava lamps and typing have been used before very successfully. One of the two popular random generation sites uses radios...

An earlier contributor suggested that the hiss was created by thermal noise in the the initial stages of the receiver. Is there any evidence to refute this claim?
 
  • #56
Shielding

The cable is a standard PC power lead with the computer connector end cut off. It's plugged into a power socket and I've bared and trapped the Earth conductor under the tin's lid. I filed off the paint on the side of the tin to ensure a good Earth contact. The Earth pin works in the power socket. There's no speaker cable, I can just hear the sound from the radio's in built speaker.
 
  • #57
Paul Uszak said:
An earlier contributor suggested that the hiss was created by thermal noise in the the initial stages of the receiver. Is there any evidence to refute this claim?

There is no evidence to support that claim, especially in your specific receiver. Also, others have said that discriminator output is NOT random. (I can't speak to evidence to support that claim). There could easily be (and probably are) design dependent characteristics of the radio that have correlative effects. You need to measure and analyze the noise. To do otherwise is poor engineering.
 
  • #58
Paul Uszak said:
An earlier contributor suggested that the hiss was created by thermal noise in the the initial stages of the receiver. Is there any evidence to refute this claim?
The noise floor is Johnson Noise. https://en.wikipedia.org/wiki/Johnson–Nyquist_noise
But if you have an antenna on the radio it will also have man made noise (QRM) and natural noise (QRN) such as lightning crashes added.

The amplitude of Johnson noise is a function of temperature and bandwidth.
What data rate in bits per second do you need from your random number generator ?
 
  • #59
Paul Uszak said:
Shielding

The cable is a standard PC power lead with the computer connector end cut off. It's plugged into a power socket and I've bared and trapped the Earth conductor under the tin's lid. I filed off the paint on the side of the tin to ensure a good Earth contact. The Earth pin works in the power socket. There's no speaker cable, I can just hear the sound from the radio's in built speaker.
Do you power the radio with the power lead? If so this is what is carrying the signal into the box. Repeat the experiment with a battery powered radio.
 
  • #60
There are all sorts of ways for non-thermal noise effects to occur in a radio-in-a-box. The RF amplifiers (least likely), the synthesizer, the mixer, the IF amplifiers, the discriminator (in FM) can all have issues. In addition to that is the likelihood (hi-probability) of low level RF signals leaking in. Maybe the audio amplifier noise gets on the power supply and creates phase jitter in the synthesizer. Especially in modern single IC AM/FM radios. Maybe a missing bypass capacitor that saved the manufacturer a tenth of a penny. A crystal that is sensitive to mechanical vibration. The list goes on and on. If you are building a toy so you can say you did it, then fine. Go for it.
 
  • #61
Shielding

This is the radio. It's battery powered. I only use the Earth conductor as a ground, which is trapped under the lid. I cut off the live and neutral conductors.
tranny.jpg
 
  • #62
meBigGuy said:
If you are building a toy...

Thanks for the additional information about noise components, but it's not a toy.

I could give you a long list of reasons why this is an entirely credible technique, but instead I offer you the following to dwell upon. A question that if you answer no to, will conclusively invalidate your cynicism once you undertake the appropriate research. Can you generate cryptographic strength random numbers from :-
1. Fish in an aquarium
2. Hitting keys on a keyboard
 
  • #63
Paul Uszak said:
Thanks for the additional information about noise components, but it's not a toy.

I could give you a long list of reasons why this is an entirely credible technique, but instead I offer you the following to dwell upon. A question that if you answer no to, will conclusively invalidate your cynicism once you undertake the appropriate research. Can you generate cryptographic strength random numbers from :-
1. Fish in an aquarium
2. Hitting keys on a keyboard
No, I can't. I don't own an aquarium. :oldcry:

More seriously, any semi-random number can be fed through a pseudo-random number function to give a random, statistically flat response. But that assumes no one will find your function and influence your semi-random number selection. Penn and Teller have some nice YouTube videos which indicate the sort of deviousness to which we humans aspire. A smarter guy will always come along.
 
  • #64
I would feel better about the fish than what you are doing with the radio.
You can do whatever you want. It makes no difference to me.
Obviously you can learn nothing from me since you already did all the appropriate research.

It's all about the noise correlation, and you have no basis for any assumptions you are making with your radio. I know how that radio works, what chip it probably uses (CXA1019S used in the ICF-S10 MK2 ), etc. It is a consumer product. If you don't validate the data, chances are it will be poor quality.

It's your choice. I've said my piece and will remain silent from here on out.
 
  • #65
Baluncore said:
What data rate in bits per second do you need from your random number generator ?

I'm not really targeting any particular generation rate. I'll take what I can get. You've caught me on the hop as I wasn't expecting to be discussing randomness extraction - I asked what makes a radio go "Shhhhh".

So, I've done a little test and by my reckoning I can get 100,000 bps.
 
  • #66
Paul Uszak said:
So, I've done a little test and by my reckoning I can get 100,000 bps.
100kbps is 5 times greater than the audio bandwidth of an FM radio and more than 10 times the bandwidth of an AM radio. So most sequential "random" bits will be the same.

The data rate you require is different from the data rate you think you might be able to get. I asked what data rate you require.

Fish in an aquarium might generate 1 bit per minute and you would probably have to keep watching. During the day they would face the light, then sleep at night.
Hitting keys on a keyboard might generate bit data at the average character rate, so long as you keep typing and can time key arrivals to better than 10usec.
 
  • #67
Paul Uszak said:
Shielding

This is the radio. It's battery powered. I only use the Earth conductor as a ground, which is trapped under the lid. I cut off the live and neutral conductors.
View attachment 84824
I think you should try grounding the box to the ground terminal of the radio instead of the Earth ground. . Perhaps one lead of the headphone jack could be used as a ground terminal.
 
  • #68
s201142010286127.gif


Typical CXA1019S design
 
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  • #69
Radio waves are magnetic lines of force and will go through almost anything without loosing all their strength. Try putting a magnet close to the outside of the box then dangle a nail on a string inside the box. Move the string and nail closer to the magnet and the string will deflect, if moved close enough the nail will attach to the side of the box. The magnet's field is constant, but radio waves are moving and cut through the antenna and induce a small detectable currant in it. This is also how a generator or alternater work, a rotating (moving) magnetic field cuts through the windings of the stater causing currant flow.
 
  • #70
Sun E Man said:
...ll go through almost anything without loosing all their strength.

How do you reconcile this thesis with the results of my experiment, whereby FM reception was totally blocked by the thin tin..?
 

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